The present invention is in the field of optical inspection techniques and relates to a method and an apparatus for inspecting patterned articles, such as integrated circuits, printed circuit boards, photolithographic masks, liquid crystal displays, etc., utilizing a collection angle design.
A typical structure of the patterned article, such as a semiconductor wafer, includes a basic cell element repeated numerous times in both lateral dimensions, creating an almost perfectly periodical two-dimensional pattern, Semiconductor wafers are inspected prior to and after a pattering procedure, since the timely detection of anomalies on the wafer""s surface is a very important factor, subsequently leading to an increase in production yields.
The prior-to-patterning inspection of wafers relies on the fact that light is scattered mainly from anomalies present on the generally flat and smooth surface of the non-patterned wafer. Thus, any detection of scattered light may be indicative of a defect. When applying an optical inspection to a patterned wafer aimed at detecting defects (e.g., the existence of foreign particles), scattered light can be caused by the pattern. Therefore, the detection of scattered light is not necessarily indicative of a defect. Conventional techniques for detecting defects are the so-called xe2x80x9cdie-to-databasexe2x80x9d and xe2x80x9cdie-to-diexe2x80x9d techniques, according to which light scattered from an individual die is compared to, respectively, the previously prepared database indicative of light scattered from the xe2x80x9cnon-defectivexe2x80x9d die and a xe2x80x9cneighborxe2x80x9d of this individual die. Differences between the signals are indicative of light scattered from anomalies present on the surface of the article. Generally speaking, detected difference in light components scattered from the individual die is indicative of the absence or addition of some features in this die, as compared to the xe2x80x9cnon-defectivexe2x80x9d die or xe2x80x9cneighborxe2x80x9d die, and is therefore considered to be a defect.
An optical inspection apparatus is typically composed of such main constructional parts as an illumination system and a light collection/detection system utilizing either a bright field or dark field mode. The bright field detection mode is based on the variability in the specular reflectance from the wafer under inspection, created by defects squandered on the wafer. The dark field detection mode uses the scattering from defects that does not orient in specular reflectance.
It is a common goal of the collection scheme to increase the signal-to-noise ratio of the detected signal as much as possible. Various forms of dark field detection schemes are found to be very effective for the purpose of defect detection. According to one known technique of this kind, disclosed in U.S. Pat. Nos. 4,898,471 and 5,604,585, to facilitate meaningful signal comparison, the light collection system collects light at one constant collection angle at the azimuth and elevation other than those where the specular reflection occurs. However, light scattered from the patterned article (e.g., wafer) always contains light components scattered from the pattern towards this collection angle, and, therefore, detection of these light components is indicated by increased xe2x80x9cnoisexe2x80x9d in the detected signal.
The present invention is aimed at improving automatic optical inspection of patterned articles by providing a novel method and apparatus enabling the signal-to-noise ratio of detected signals to be significantly increased.
The present invention takes advantage of a technique utilizing a variable angle design of the light collection system. This technique is disclosed in a co-pending application assigned to the assignee of the present application. According to this technique, an article (e.g., wafer) under inspection is scanned region-by-region, light scattered from each of the scan regions is collected with a certain maximum collection angle constant for each scan region, and directed towards a detection means trough a filter. The latter is selected such as to pick up from the entire collected light only that part thereof propagating with a solid angle segment of the entire collection angle, where the intensity of light scattered from the pattern is minimal, as compared to other solid angle segments of the maximum collection angle. This enables to increase the signal-noise ratio of the detected signal. The detection means comprises at least one detection unit operating in a dark field imaging mode, i.e. collecting light components scattered from the article at the azimuth and elevation different from those where the most specular reflection occurs.
The main idea of the present invention consists of selecting the variable angle solution. This is implemented by acquiring a bright field or high resolution dark field image and analyzing the acquired image to determine a solid angle segment of the propagation of light returned from the pattern within the collection angular field. This enables customized light collection (CLC) to be applied to dark field scattering signals, so as to prevent that part of the scattered signals, which is associated with the pattern on the article and constitutes xe2x80x9cnoisexe2x80x9d, from reaching the detector, thereby allowing the detection of only that part of the scattering signals which is associated with any feature in the illuminated region other than those of the pattern. In other words, a suitable mask is placed in the optical path of light scattered from the article and propagating towards a dark field detector, such that the mask cuts off the solid angle segment of propagation of light scattered from the pattern. This results in a significant increase in the signal-to-noise of the detected signals.
Thus, according to one aspect of the present invention, there is provided a method for optical inspection of a patterned article, the method comprising the steps of:
(i) illuminating a region on the article with incident light to produce light returned from the illuminated region;
(ii) acquiring an image of the illuminating region, and generating data representative thereof;
(iii) analyzing the generated data and determining intensity distribution of light components scattered from the pattern of the illuminated region within a certain collection angular field outside a solid angle of propagation of specularly reflected light;
(iv) based on the determined distribution, filtering light collected with said certain collection angular field, so as to collect light components scattered from the illuminated region and propagating with at least one predetermined solid angle segment of said certain collection angular field, and to direct the collected light components to a detection unit.
The term xe2x80x9ccollection angular fieldxe2x80x9d used herein signifies a maximum solid angle of collection of light scattered from the article defined by an optical arrangement of the detection unit.
In step (ii) above, light detected for acquiring the image of the illuminated region may be that specularly reflected from the illuminated region (i.e., bright field detection mode) or the scattered light propagating with a solid angle outside the solid angle of propagation of specularly reflected light (i.e., dark field detection mode). To obtain data indicative of the acquired image of the pattern in the illuminated region, sufficiently high resolution should be provided. To his end, the light collecting optics utilizes a high numerical aperture objective lens.
Analysis of the generated data consists of the so-called xe2x80x9cmodelingxe2x80x9d of the pattern structure, and obtaining a discrete two-dimensional (or three-dimensional) array indicative of the scattering pattern. This data is used to simulate the dark-field scattering pattern within the collection angular field. The simulation results enable to determine the xe2x80x9cbackgroundxe2x80x9d intensity rising from the periodic pattern as the intensity lobe in the simulation/imaging plot, and collect light components propagating with the solid angle segment(s) of the collection angular field other than that of the xe2x80x9cbackgroundxe2x80x9d light components. This is implemented by locating an appropriate mask (filter) in an appropriate position within the optical path of the entire collected light (the so-called Fourier filtering).
It should be understood that each specific pattern structure is characterized by the solid angle segment of propagation of the xe2x80x9cbackgroundxe2x80x9d light components scattered from the periodic pattern. Hence, a library (database) can be previously designed containing a plurality of data records, each being indicative of the solid angle segments of the propagation of light scattered from a specific periodic pattern, with respect to a plurality of different pattern structures. These different pattern structures may be those produced at different manufacturing steps of a specific article (e.g., wafer), or may be associated with a plurality of different article types. This library actually presents reference data to be used for collecting the light components scattered from the illuminated region with said at least one predetermined solid angle segment of the certain collection angular field.
Thus, according to another aspect of the present invention, there is provided a method for inspecting a patterned structure with a certain maximum solid angle of collection of light scattered from the article to be inspected, said certain maximum solid angle of collection lying outside a solid angle of propagation of light specularly reflected from said patterned structure, the method comprising the steps of:
providing a database containing data representative of a plurality of solid angle segments of light propagation within said certain maximum solid angle of collection, said plurality of solid angle segments corresponding to a plurality of patterned structures including said patterned structure to be inspected, each of said solid angle segments corresponding to a direction of propagation of light components scattered from the corresponding patterned structure where the contribution of light scattered from the pattern is substantially minimal as compared to other solid angle segments of the solid angle of collection;
selecting from said database the data representative of said patterned structure to be inspected, which data is to be used for filtering from the entire light collected with said certain maximum collection angle the light components propagating with the corresponding solid angle segment, and allowing the detection of said light components.
The provision of the database utilizes detection of light scattered from each patterned structure in the plurality of structures (by bright-filed or high resolution dark-field detection mode), and analyzing data indicative thereof. The analysis procedure includes the steps of: modeling an image of the pattern in the form of a two- or three-dimensional array of discrete scatterers, determining the intensity distribution of light scattered from this modeled pattern within the maximum solid angle of collection.
According to yet another aspect of the present invention, there is provided an optical inspection apparatus for inspecting a patterned article, the apparatus comprising:
an illumination system for illuminating a region on the article;
a light collection system collecting light components scattered from the illuminated region at a certain collection angular field located outside a solid angle of propagation of specularly reflected light components,
a filter operable in the optical path of the collected, scattered light components such as to separate therefrom light components propagating with a predetermined solid angle segment of said certain collection angular field;
a detection system comprising a detection unit for receiving a filtered part of the collected, scattered light and generating data representative thereof; and
a control unit operating the filter in the optical path of the collected scattered light in accordance with predetermined data indicative of at least one solid angle segment of propagation of light scattered from the pattern in the illuminated region.
The predetermined data can be previously obtained either by the same optical inspection apparatus or by another inspection apparatus utilizing the bright field or high resolution dark field detection mode.
Preferably, the filter is in the form of a mask assembly including a plurality of different masks, each for filtering light components propagating with at least one specific solid angle segment of the maximum collection angular field, or corresponding to a specific pattern structure. Such a mask assembly can be selectively operated to locate the selective one of the masks in the optical path of the collected light. A mask can be of an LCD, mechanical, etc. type. The filter is preferably placed in the Fourier plane (conjugate plane) of the article plane.
The invention also provides, according to yet another its aspect, an optical inspection apparatus for inspecting a patterned article, the apparatus comprising:
(a) an illumination system for illuminating a region on the article;
(b) a light collection system comprising a first light collection arrangement collecting light components returned from the illuminated region, and a second light collecting arrangement collecting light components scattered from the illuminated region at a certain collection angular field located outside a solid angle of propagation of specularly reflected light components,
(c) a filter selectively operable in the optical path of the collected, scattered light components for separating therefrom light components propagating with a predetermined solid angle segment of said certain collection angular field;
(d) a detection system comprising a fist detection unit that detects the light components collected by the first light collection arrangement and generates data representative thereof, and a second detection unit that detects a filtered part of the collected, scattered light and generates data representative thereof; and
(e) a control unit, which is responsive to the data generated by the first detection unit for analyzing said data and operating the filter accordingly.
According to yet another aspect of the present invention, there is provided a control unit to be used in a system for optical inspection of a patterned structure with a certain maximum solid angle of collection of light scattered from the structure to be inspected, said certain maximum solid angle of collection lying outside a solid angle of propagation of light specularly reflected from the structure, the control unit comprising a memory that stores a database containing a plurality of data records corresponding to a plurality of patterned structures including said patterned structure to be inspected, wherein each data record contains data indicative of at least one solid angle segment of said certain maximum solid angle of collection corresponding to a direction of propagation of light components scattered from the corresponding patterned structure where the contribution of light scattered from the pattern is substantially minimal, as compared to other solid angle segments of the maximum solid angle of collection.
In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:
FIG. 1 schematically illustrates the main components of an optical inspection apparatus according to one embodiment of the invention;
FIG. 2 illustrates an optical inspection apparatus according to another embodiment of the invention;
FIG. 3 schematically illustrates the main components of a detection unit of the apparatus of FIG. 1;
FIG. 4 schematically illustrates the construction of light collecting optics of the detection unit of FIG. 2;
FIG. 5 schematically illustrates an example of a mask assembly suitable to be used as a filter in the apparatus of FIG. 1;
FIGS. 6a and 6b illustrate the real bright field image of an illuminated region, and an xe2x80x9cidealizedxe2x80x9d, simulated image thereof, respectively;
FIGS. 7a to 7e illustrate simulated images of five different pattern structures, respectively;
FIG. 8 schematically illustrates an illumination function projected on a scattering plane, used to simulate a Gaussian beam impending on the surface of an article;
FIG. 9 schematically illustrates the geometry of the scattering problem used for simulation purposes; and
FIGS. 10a and 10b illustrate, respectively, the simulation scattering results and imaging of the intensity profile.